Heterogeneous precipitation

The text now turns to the heterogeneous precipitation processes which, as depicted in Figure 5.25, are grouped into two types electrochemical and electrolytic. 

It can be concluded that in principle the heterogeneous precipitate membrane electrodes act in the same way as the corresponding homogeneous electrodes, but often they are slower in response in practice however, they still offer manufacturing possibilities where suitable pellets of the pure crystalline material cannot be obtained. 

If catalysts are prepared by coprecipitation, the relative solubilities of the precipitates and the possibility for the formation of defined mixed phases are essential. If one of the components is much more soluble than the other, there is a possibility that sequential precipitation occurs. This leads to concentration gradients in the product and less intimate mixing of the components. If this effect is not compensated by adsorption or occlusion of the more soluble component, the precipitation should be carried out at high supersaturation in order to exceed the solubility product for both components simultaneously. Precipitation of the less soluble product will proceed slightly faster, and the initially formed particles can act as nucleation sites for the more soluble precipitate which forms by heterogeneous precipitation. The problem is less crucial if both components form a defined, insoluble species. This is for instance the case for the coprecipitation of nickel and aluminum which can form defined compounds of the hydrotalcite type (see the extensive review by Cavani et al. [9] and the summary by Andrew [10]). 

These problems are avoided if a continuous process is employed for the precipitation however, this makes higher demands on the process control. In a continuous process all parameters as temperature, concentrations, pH, and residence times of the precipitate can be kept constant or altered at will. Continuous operation is, for instance, used for the precipitation of aluminum hydroxide in the Bayer process. Bayer aluminum hydroxide is the main source for the production of cata-lytically active aluminas. The precipitation step of the Bayer process is carried out continuously. An aluminum solution supersaturated with respect to Al(OH)3, but not supersaturated enough for homogeneous nu-cleation, enters the precipitation vessel which already contains precipitate so that heterogeneous precipitation is possible. The nucleation rate has to be controlled very carefully to maintain constant conditions. This is usually done by controlling the temperature of the system to within 2-3 degrees [7]. 

When such strategies are used to produce approximately homogeneous solids, the other phenomenon mentioned above can occur, namely heterogeneous precipitation of one compound on the surface of the already formed precipitate of another phase. This is shown in Fig. 3, for the precipitation of compound B after A has already precipitated. This has given rise to severe misinterpretations of physico-chemical characterization of precipitated solids, especially when surface-sensitive techniques such as X-ray photoelectron spectroscopy (XPS) were used. 

The development of tightly cemented lenses is due to the heterogeneous precipitation of marine fringe cement. That fringe cementation was among the first diagenetic processes is evident from the para-... 

A sintered-glass funnel is best, as the precipitate is very fine. Alternatively, if a sufficiently large funnel of this type is not available, a Buchner funnel may be used but then the heterogeneous precipitate must be dried in vacuo. 

Matsubara I, Paranhaman M and Allison S (2000) Preparation of Cr-doped YiAl Oi2 phosphors by heterogeneous precipitation methods and their luminescence properties. Mater Res Bull 35 217 — 224. 

N. Kamp, A. Sullivan, R. Tomasi, and J.D. Robson, Modelling of Heterogeneous Precipitate Distribution Evolution during Friction Stir Welding Process, Acta Mater., Vol 54, 2006, p 2003-2014... 

Solute depletion due to slow volume diffusion coupled with fast grain boundary diffusion and heterogeneous precipitation of phases. 

This situation requires heterogeneous nu-cleation of the p phase on a grain boundaries. Preferred locations of heterogeneous precipitation include grain boundaries, impurity particles, and dislocations [25]. 

A more sophisticated and appropriate description of the depletion of a beneficial alloying element when precipitates are formed on grain boundaries is described by the coUector-plate mechanism and other more advanced treatments. This mechanism describes heterogeneous precipitation of a second-phase precipitate... 

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